There is an array of pixels in a full-color organic display (also known as an organic light-emitting display). Each pixel can include red, green, and blue electroluminescence (EL) subpixels (commonly referred to as light-emitting subpixels, RGB subpixels, or RGB elements). Each light-emitting subpixel consists of a basic organic light-emitting diode (OLED). The basic OLED has in common an anode, a cathode, and an organic EL medium (or EL unit) sandwiched between the anode and the cathode. The organic EL medium can include one or more layers of organic thin films, where one of the layers is primarily responsible for EL or light emission. This particular layer is generally referred to as the light-emitting layer (LEL) of the organic EL medium. Other organic layers present in the organic EL medium can primarily provide charge transport functions and are referred to as either the hole-transporting layer (HTL) or the electron-transporting layer (ETL). Tang et al. demonstrated highly efficient OLEDs in “Organic Electroluminescent Diodes”, Applied Physics Letters, 51, 913 (1987) and in commonly assigned U.S. Pat. No. 4,769,292. Since then, numerous OLEDs with alternative layer structures have been disclosed, and many different types of EL materials have also been synthesized for use in OLEDs. In forming the pixels in a full-color organic display, it is also necessary to apply a method to precisely pattern the LEL of the organic EL medium or the entire organic EL medium. In commonly assigned U.S. Pat. No. 5,937,272, Tang has taught a method of patterning multicolor EL subpixels onto a thin-film-transistor (TFT) array substrate by vapor deposition of the EL materials. Therefore, combining the basic OLED structure with organic EL materials, a precision patterning method, and a driving circuitry, a full-color organic display can be realized.
Lifetime of a full-color organic display is very important for display applications. The lifetime of a full-color organic display is mainly determined by the lifetime of the EL subpixels. Specifically, the lifetime of a full-color organic display is generally determined by the EL subpixel color that has the shortest lifetime within each of the pixels. The lifetime of an EL subpixel is defined as the time to reach half the initial luminance at a given current density. The lifetime of a blue EL subpixel is shorter than that of a green EL subpixel and even much shorter than that of a red EL subpixel. Obviously, the lifetime of a full-color organic display is limited by the blue EL subpixel. Therefore, improving the lifetime of the blue EL subpixels will have a large impact on display applications.
There are several ways to improve the lifetime of the blue OLEDs. For example, Shi et al. in “Anthracene Derivatives for Stable Blue-Emitting Organic Electroluminescence Devices”, Applied Physics Letters, 80, 3201 (2002) and Hosokawa et al. in U.S. patent application Ser. No. 2003/0077480 A1, achieved improved operational stability of blue emission by selecting proper materials. On the other hand, Yamada in U.S. Pat. No. 6,366,025 B1 and Cok et al. in commonly-assigned U.S. patent application Ser. No. 10/315,622, entitled “Color OLED Display Having Repeated Patterns of Colored Light Emitting Elements” disclosed a full-color organic display having EL subpixels with different surface emitting areas, wherein the surface emitting areas of the EL subpixels are selected to extend the lifetime as well as to achieve better white balance through combined red, green, and blue emission.
From the operational point of view, the lifetime of an EL subpixel is dependent on the drive current density. A blue EL subpixel with a relatively large surface emitting area will need less current density to achieve the same brightness as a blue EL subpixel with a smaller surface emitting area. Therefore, the lifetime can be expected to increase. However, in a full-color display, the total surface emitting area of each pixel is predetermined. If a blue EL subpixel within a pixel occupies a relatively large surface emitting area, the red and green EL subpixels would then have to occupy relatively small surface emitting areas. This means the red and green EL subpixels have to be driven at a relatively high current density to reach a certain brightness. As a result, emitting at the same brightness, the red and green EL subpixels with smaller surface emitting areas will have shorter lifetimes than those red and green EL subpixels with larger surface emitting areas. Therefore, the lifetime improvement of the blue EL subpixel is at the expense of the lifetime of the red and green EL subpixels.